1. Field of the Invention
The present invention relates to an apparatus and method for automatically centering and focusing a corneal image on a detection array of a corneal topographer.
2. Description of the Related Art
Photokeratoscopes, and more recently, corneal topographers, are used to determine the contour of the cornea of the human eye, thus facilitating the design and fitting of contact lenses as well as for use in the performance of surgical procedures.
For example, contact lenses are fitted on the basis of the measurement of the central corneal curvature using a keratometer or an ophthalmometer. It is thus necessary to select lenses from different trial sets until the "quasi" appropriate one is found. This is a tedious procedure that requires a long time and a wide variety of trial cases. Contact lenses, thus fitted, require various modifications after they are ordered from the laboratory to be adapted to each cornea. Because of imprecision of the geometry and poor quality of the finished lenses, some patients could not tolerate them. If a contact lens is to be designed on the spot and without a return trip of the patient, there is a need for permitting the images to be processed and a determination to be made as quick as possible.
Corneal topographers allow data to be processed more efficiently, which is urgently needed in the performance of certain surgical procedures. One such surgical procedure is known as radial keratotomy, where microincisions are placed on the cornea in an attempt to surgically modify the curvature of the cornea, and thereby reduce or eliminate myopia or astigmatism. Thus, the accuracy of this procedure depends to some degree on the ability to measure the original shape of the cornea. Improvements on corneal topographer instrumentation is invaluable in continuing to increase the accuracy of radial keratotomy or refractive surgery.
Corneal topography apparatus is also useful in myopic and hyperopic keratomileusis in which the corneal curvature is altered to improve refractive error by removal of the corneal "disc" predicted thickness with a microkeratome. Corneal transplantation is performed for scarred or diseased corneas by replacing optically inferior cornea with clear tissue. Sutures are placed strategically so that the tissue heals in the most spherical fashion without astigmatism. Corneal topography apparatus is further useful in intraocular removal of a cloudy lens and subsequent replacement of an intraocular lens, wherein, after entering the eye, the wound and sutures are manipulated so as to reduce astigmatism. Corneal topography provides the surgeon with information how to close the wound and how to calculate the power of the intraocular lens.
In conventional photokeratoscopes used for measuring the contour of the cornea, concentric rings of light from a source of light within a housing are directed onto a cornea and reflected by the cornea onto the film of a camera as an image of the rings. The deviation of the rings from their known concentricity can be measured on the film and this data processed mathematically to determine the actual contour of the cornea, which of course, is not a perfect sphere and which differs from one individual to another. Conventional photokeratoscopes are disclosed in U.S. Pat. Nos. 3,248,162 and 3,598,478.
In use of apparatus of this type, the cornea of an individual to be examined is located in a position having its optical axis at least approximately aligned with the axis of the target of rings and the lens of the camera. The deviation of this optical axis from alignment can then be determined. Although the calculations for determining its contour do not require absolute alignment, it is desirable that it be as closely aligned as possible in the interest of a more accurate determination. In order to avoid relocating the cornea of the individual to be examined, and to bring the axis of the images rings into substantial alignment with the optical axis of the eye, the target and optical system of the camera are preferably adjustable with respect to the chin rest. French Patent No. 7122413 shows a photokeratoscope in which such adjustment may be made by a "joy stick" conventionally located for use by the operator of the photokeratoscope.
A corneal topographer is disclosed in U.S. Pat. No. 4,978,213, which provides significant improvements over prior photokeratoscopes used for corneal measurements. The corneal topographer or the '213 patent generally includes a housing including an apparatus for providing illuminated rings for reflection off the cornea. Camera means, such as a charge-coupled-device (CCD) camera system, is included for sensing the images of rings of light reflected from the cornea. The camera apparatus sends standard video signals to a computer including a conventional, off-the-shelf image processor, which digitizes the video signals. The computer analyzes the digital data and produces data useful in determining a contour of the cornea of the human eye. Positioning means, preferably comprising step motors, is provided for manually positioning the corneal topographer to receive accurate measurements.
The effectiveness and accuracy of the measurements taken from a corneal topographer is directly dependent upon the accuracy of the positioning of the apparatus relative to the cornea. The correct mapping of the eye depends directly upon the centered image, since misalignment of the equipment results in skewed measurements, and thus inaccurate mapping and calculations. If the image is misaligned, the corneal contour may not be accurately determined, leading to erroneous diagnosis in surgical procedures or fitting lenses. It is further recognized that an unfocused image increases error in the corneal data measurements. The better the focus, the more accurate the measurements and the better the diagnosis can be made from those measurements.
It has been discovered that manual focusing and centering by an operator is inherently inaccurate and time consuming. The level of accuracy and typically varies from one operator to the next, and also on the amount of care used by any given operator. Operator error is particularly problematic since it is based on upon subjective judgment which varies from one operator to the next and even with the same operator over time. Also, even an experienced operator of corneal topographers must adjust and readjust the image to achieve proper centering and focusing. Meanwhile, the patient must remain as still as possible, since even slight eye movements requires readjustment by the operator. Manual positioning has proved to be a tedious and frustrating experience, especially with a fidgety patient.
Thus, it is desirable to substantially reduce the amount of time required for, and operator error typically resulting from, manually positioning corneal topography equipment. Improvements in the accuracy of the corneal measurements will improve the diagnosis based upon those measurements.